private void button1_Click(object sender, EventArgs e)
         {
            //打开FontDialog 选择字体样式并保存
             FontDialog fd = new FontDialog();
            fd.ShowColor = true;
            if (fd.ShowDialog() == DialogResult.OK)
            {
                object[] obj = new object[] { fd.Font, fd.Color };
                BinaryFormatter formatter = new BinaryFormatter();
                using (FileStream stream = File.Open("aa.dll", FileMode.Create))
                {
                    formatter.Serialize(stream, obj);
                }
            }
            
        }

        private void button2_Click(object sender, EventArgs e)
        {
            object[] obj;
            BinaryFormatter formatter = new BinaryFormatter();
            using (FileStream stream = File.Open("aa.dll", FileMode.Open))
            {
                obj = (object[])formatter.Deserialize(stream);
            }
            Font font = (Font)obj[0];
            Color color = (Color)obj[1];

            //读取保存的字体样式信息并设置
            this.textBox1.Font = font;
            this.textBox1.ForeColor = color;
           
        }

------解决方案--------------------

formload 事件读取处加上判断
C# code
youPath1 = Application.StartupPath + @"\\文字格式.dll";
if (File.Exists(youPath1))
{
    object[] obj;
    BinaryFormatter formatter = new BinaryFormatter();
    using (FileStream stream = File.Open(youPath1, FileMode.Open))
    {
        obj = (object[])formatter.Deserialize(stream);
    }
    Font font = (Font)obj[0];
    Color color = (Color)obj[1];
    txt1.Font = font;
}